In the paper industry, crêping doctor blades are used for the manufacture of tissue and other related paper products. Creping doctor blades may be made of different materials such as Carbon steel strip, tool steel strip, composites and polymers. In addition, different types of coating may be applied by Thermal Spray coating in order to reinforce the working edge of the crêping doctor blade such as described in U.S. Pat. No. 7,244,340 B2.
In many Tissue mills Ceramic coated blades are the chosen crêping doctor blades due to the fact that they have relatively high hardness and therefore wear resistance but also they can accommodate the high temperatures that result from operation in contact with a steam filled ‘Yankee’ cylinder. In addition, they are not susceptible to one of the major wear mechanisms that limits the useful life of steel blades i.e. adhesive wear. The service life of ceramic coated crêping doctor blades is longer than even the best hardened steel blades but is still not consistently as long as is required for modern efficient tissue mills. This can be due to limitations in the hardness that can be achieved with ceramic coatings and also by the relationship between hardness and toughness, where extremely hard coatings can be prone to chipping at the important working edge of the blade.
One further drawback of ceramic coated crêping doctor blades is the fact that they are usually deposited by a plasma spraying process and therefore need a soft bond coat such as Ni—Cr. During the life of the blade, as wear proceeds, this bond coat can become exposed on the surface upon which the web impacts during crêping, leading to the need to make adjustments to the machine set-up to maintain paper quality.